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For customers who
are new to single-cylinder performance, this page will explain basic carburetor
operation, identify carburetor circuitry and give you a rudimentary knowledge of
a carburetor and how it works.
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Motorcycle
carburetors described here are of the variable venturi type. With
this type of carburetor, the maximum depression zone is beneath the
throttle valve (slide) which is raised and lowered by the throttle cable,
controlling the speed of the engine.
As shown in
the drawing, the bottom of the slide features a tapered needle which fits
into the fuel pick-up tube (needle jet) to meter the fuel delivery of the
tube from about 1/4 throttle to 3/4 throttle. From 3/4 throttle to
full throttle, the needle will not affect the fuel flow. At this
point, fuel flow is metered by the main jet (position at the bottom of the
tube). |
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This
description is a simple model, shown to give you a basic idea of how a
carburetor functions. There are variations on this design, but all
work using similar principles. |
Sudco
Corporation has an excellent website, which has exploded views of many popular
OEM and aftermarket carburetor styles. Most of the diagrams and
information listed here come from the Sudco website.
Click on the diagram above, and print off a copy of this view so you can refer to
it (print hint: make sure to click on the option for your printer that says
'scale to fit' so the picture fits on one piece of paper).
This carburetor design is used on many four
stroke (and early two stroke) models, and is similar
to many others. Although a parts list is not included with the exploded
view, we have included a list of these parts below. Proper names are in
CAPS, commonly used names are in (parenthesis). Jets which are covered in
the trouble shooting section of this
website are HIGHLIGHTED.
| Sudco # |
Description |
|
Sudco # |
Description |
| 1 |
THROTTLE CABLE, RUBBER CAP |
|
22-25 |
FLOAT BOWL ASSEMBLY - Some are adjustable,
and may be tuned to allow more or less fuel into the bowl. |
| 2-3 |
CABLE ADJUSTER & CABLE ADJUSTER LOCKNUT -
Used to take up or remove slack in a throttle cable. |
|
26-27 |
MAIN JET PLUG & WASHER - Remove this to
access the main jet without disassembling the carburetor. |
| 4-5 |
MIXING CHAMBER TOP (Carb Cap) & MIXING
CHAMBER GASKET (Carb Cap Gasket) |
|
28 |
AIR JET - Meters air flow into the NEEDLE JET
(#11). Acts as a fine tuning component to regulate the fuel-air
mixture. |
| 6 |
THROTTLE VALVE SPRING - Returns the throttle
(and the slide) to the closed position. |
|
29 |
AIR ADJUSTING SCREW (Air Screw) - Adjusts air
into the passage through the pilot jet. Turn out for leaner mixture
and in for richer mixture. In some carburetors, this is an air/fuel
mixture screw and the tuning is opposite. Typically set 1
1/2 to 2 turns out. |
| 7 |
SPRING SEAT PLATE |
|
32 |
IDLE ADJUSTING SCREW
(Idle Screw) - Adjusts engine RPM at idle. Some OEM
carburetors incorporate this into the choke mechanism. |
| 8 |
NEEDLE POSITIONING CLIP (Clip) - Used to
richen or lean by moving up (leaner), or down (richer). Changes
jetting from 1/4 to 3/4 throttle. |
|
30 & 31 |
SPRINGS FOR AIR AND IDLE ADJUSTERS |
| 9 |
JET NEEDLE (Needle) - Meters fuel from 1/4
throttle to 3/4 throttle. Can be adjusted using NEEDLE POSITIONING
CLIP (#8 listed above). |
|
33 |
NEEDLE VALVE WASHER |
| 10 |
THROTTLE VALVE (Slide) - Meters air/fuel
mixture between 1/8th and 1/2 throttle. Typically no changes are
needed on an OEM throttle valve when rejetting. |
|
34 |
NEEDLE VALVE & SEAT - Regulates fuel
flow from gas tank to float bowl. If this is worn or pitted, engine
can run erratically. |
| 11 |
NEEDLE JET - This is the jet which the JET
NEEDLE (#9) drops down into. Affects how the JET NEEDLE
operates. Typically no NEEDLE JET changes are required when
rejetting. |
|
35 |
MAIN JET RING & WASHER |
| 12-17 |
STARTER PLUNGER ASSEMBLY (Choke) |
|
36 |
MAIN JET -
Affects jetting from 3/4 to full throttle |
| 18 |
MIXING CHAMBER BODY (Carb Body) |
|
37 |
VENT TUBE ANCHOR PLATE |
| 19-20 |
FLOAT BOWL GASKETS & BAFFLE PLATE |
|
38 |
FLOAT BOWL SCREW |
| 21 |
PILOT JET - Meters fuel from 0 to 1/4
throttle |
|
39-45 |
Not used in motorcycle or ATV carburetors |
Below are two short tutorials showing the most common carburetor styles, and how to
remove the mainjet and needle from each of them.
BIG HUGE NOTE
- Save yourself time and headaches! Never work on a carburetor in a
messy garage, especially around open containers full of parts. Carburetors
have many small pieces which have a tendency to go zinging off into space.
Allowing yourself a large clean area gives you a greater likelihood of finding
the needle clip or mainjet ring when it leaves in a hurry!
|
MIKUNI 28mm ROUND SLIDE CARB
This design is similar to most round and flat slide carburetors.
In this design, the throttle attaches directly to the slide through the
top of the carb. The cap screws off. |
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| ACCESSING THE MAINJET
The mainjet is accessed from the bottom of the carburetor by removing
the mainjet plug (Note: some smaller models do not have a plug, if
this is the case, you will need to remove the entire bowl).
If there isn't enough clearance to get a tool in
to remove the lower bolt, you will need to loosen the hose clamps holding
the carb to the airbox and the intake manifold, and rotate the carburetor
(May not be possible on some models, which are bolted to the manifold). This will allow you to access the mainjet plug.
Before removing the plug, TURN OFF THE FUEL. All
of the fuel in the bowl will flow out when you have the plug removed - you
may want to wipe it off the engine or it may remove some paint.
|
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| REMOVE THE MAINJET
A mainjet wrench is the easiest thing to use to remove the mainjet.
Since they only cost a few dollars at your local shop, it's a good idea to
pick one up.
On some carburetors, there is a ring (brass or plastic) around the
mainjet, this ring will fall out when the jet is removed, be careful not
to lose it. |
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| MAINJET - This view shows the mainjet (on the
right), the mainjet ring (center), and a basic jet wrench (left). |
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| REMOVING THE NEEDLE
By unscrewing the cap, you will be able to pull out (from left to
right) the cap, spring, spring seat plate, slide and needle with clip
attached.
Detach the cable from the slide in order to access the needle and
clip. |
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| VIEW AFTER REMOVING THE CAP
From this view, you can see the NEEDLE JET. You
can also see the IDLE ADJUSTING SCREW poking into the slide bore at the 6
o'clock position. |
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| NEEDLE
The needle will have groves, which the E CLIP fits into. By
moving the clip down (towards the tapered end of the needle), you will
lift the needle and RICHEN your jetting. Moving the clip UP will
drop the needle and LEAN the jetting. Removing and
Replacing the clip: If you've never removed a
needle clip before, the easiest and safest way to remove the clip is to
work in a clean uncluttered area (if the clip flies into space, you have
a better shot at finding it!). Lay the needle on a clean cloth or
paper towel with the clip opening facing down. Putting fingers on
the needle on either side of the clip, and press down gently. The
clip will pop off. You can re-install the clip
the same way, or by using a pair of needle-nose pliers to gently press
the clip back in the selected groove. Be careful
not to bend or scratch the needle. |
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| KEIHIN 29mm BUTTERFLY STYLE CARBURETOR
This particular carburetor is from a TRX 250X, but the design is
similar to most of the modern butterfly or pumper carbs. |
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| ACCESSING THE MAINJET
The mainjet is accessed from the bottom of the carburetor by removing
the mainjet plug (see arrow).
If there isn't enough clearance to get
a tool in to remove the lower bolt, you will need to loosen the hose
clamps holding the carb to the airbox and the intake manifold, and rotate
the carburetor. This will allow you to access the mainjet plug.
Before removing the plug, TURN OFF THE FUEL. All of the fuel in
the bowl will flow out when you have the plug removed. |
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VIEW OF MAINJET IN CARBURETOR
A mainjet wrench is the easiest thing to use to remove the mainjet.
Since they only cost a few dollars at your local shop, it's a good idea to
pick one up.
On some carburetors, there is a ring (brass or plastic) around the
mainjet, this ring will fall out when the jet is removed, be careful not
to lose it. |
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| MAINJET REMOVED
Here you can see the mainjet holder. Note the holder is offset from the
mainjet plug hole. This is normal. |
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| REMOVING THE NEEDLE
The needle is accessed from the top of the carburetor. With this
style of carburetor, the throttle cable enters through the side, and does
not directly lift the slide.
Modern style butterfly, CV and pumper carbs are the typically the same
design. |
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| CARBURETOR TOP REMOVED
Once the top is removed, you will see some sort of linkage, which
allows the throttle action to be transferred to the slide. In this
case, the linkage is held in place by a screw at the top of the pivot (a),
and two screws on top of the slide (not visible in this view). |
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| First, remove the screw holding the pivot arm in place. |
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| Next, remove the screws holding the pivot arm to the
slide. Flip the pivot arm up and away from the slide. |
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| PIVOT ARM REMOVED
Now you can view the slide. Remove it to access the needle. |
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| The needle is easily removed from the slide. |
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| NEEDLE
The needle will have groves, which the E CLIP fits into. By
moving the clip down (towards the tapered end of the needle), you will
lift the needle and RICHEN your jetting. Moving the clip UP will
drop the needle and LEAN the jetting. Removing and
Replacing the clip: If you've never removed a
needle clip before, the easiest and safest way to remove the clip is to
work in a clean uncluttered area (if the clip flies into space, you have
a better shot at finding it!). Lay the needle on a clean cloth or
paper towel with the clip opening facing down. Putting fingers on
the needle on either side of the clip, and press down gently. The
clip will pop off. You can re-install the clip
the same way, or by using a pair of needle-nose pliers to gently press
the clip back in the selected groove. Be careful
not to bend or scratch the needle. |
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| GASKET CARE - Before replacing the carburetor top, make sure
the gasket is in good condition and is installed in the groove. |
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After
getting a basic idea of how a carburetor works, return to the
Jetting
page for information on how to make it work better!
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